This invention relates to optical sheet material and, more specifically, to such sheet material characterized by light diffusion properties. This invention also relates to back light displaying using the optical sheet, and to processes for making the sheet material.
In many current displays systems, for example in Liquid Crystal Displays (LCD), it is desirable to have diffusing components. Examples of the utility of diffusing components include (but are not limited to) masking artifacts, such as seeing electronic components located behind the diffuser film and/or hiding injection molded patterns or printing on the light guide pipe of the display, improved uniformity in illumination and increased viewing angle. In a typical LCD display, diffusion of light is introduced into the backlight assembly by adding separate films (i.e., a stack) that are comprised of a non-diffusing substrate to which a highly irregular, diffusing surface treatment is applied or attached. It is thus desirable to generate diffuse light without the added cost of separate films.
In backlight computer displays or other display systems, optical films or sheet material are commonly used to direct, diffuse or polarize light. For example, in backlight displays, brightness enhancement films (BEFs) use prismatic structures on the surfaces thereof to direct light along a viewing axis (i.e., an axis normal to the display). This enhances the brightness of the light viewed by the user of the display and allows the system to consume less power in creating a desired level of on-axis illumination. Such films can also be used in a wide range of other optical designs, such as in projection displays, traffic signals, and illuminated signs.
In current displays systems, for example in Liquid Crystal Displays (LCD), it is desirable to have diffusing components. Examples of the utility of diffusing components include (but are not limited to) masking artifacts, such as seeing electronic components located behind the diffuser film and/or hiding injection molded patterns or printing on the light guide pipe, improved uniformity in illumination and increased viewing angle. In a typical LCD display, diffusion of light is introduced into the backlight assembly by adding separate films (i.e., a stack) that are comprised of a non-diffusing substrate to which a highly irregular, diffusing surface treatment is applied or attached. It is thus desirable to generate diffuse light without the added cost of separate films.
Some of undesirable features of diffuser films are a lack of dimensional stability during high temperature and humidity testing due to mismatched material properties between the substrate and coating, coating defects due to lack of coating process robustness, defect generation during material conversion due to scratches in the coatings, impressions or texture non-uniformities due to tooling and/or web handling rollers, dust attraction and film stickage due to static charge creation, and reduced luminance and luminance uniformity. The first four of the above undesirable features may reduce the yield during the various cutting and assembly processes. It is desirable that diffusers minimize the above undesirable features while enhancing the desirable features of high brightness and brightness uniformity.